In recent years, being solvent-free and water-based has been required also with respect to recording materials and inking materials utilized for such as a printer, a printing-machine, a marker and a writing tool. Particularly, as water-based recording materials, widely utilized are those comprising primarily an aqueous solution of a water-soluble dye or micro-dispersion of a pigment.
As a water-based ink employing a water-soluble dye, mainly utilized are those comprising an aqueous solution of a water-soluble dye classified into such as acid dyes, direct dyes and a part of food dyes being added with a glycol series or a alkanolamine series as a moisture retaining agent, a surfactant to control the surface tension, and appropriately a viscosity increasing agent. Such a water-based ink employing a water-soluble dye is most generally used with respect to high reliability causing no clogging at a pen tip or in a printer, however, it cannot be helped that the application purpose is limited and recording quality is deteriorated due to easy blotting on recording paper. That is, a water-soluble dye, which immerses into a recording paper to be dried and adhered, is hardly said to be dyed resulting in exhibiting very poor light fastness.
Further, to solve a problem that a a water-based ink employing a water-soluble dye is poor in water resistance and light fastness, a proposal in which a water-dispersible resin is colored by an oil-soluble dye or a hydrophobic dye has been made as ink for ink-jet recording. For example, ink utilizing emulsion polymerization particles being dyed with an oil-soluble dye or dispersed polymer particles is proposed in such as JP-A Nos. 55-139471, 58-45272, 3-250069, 8-253720, 8-92513, 8-183920 and 2001-11347 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public inspection). Attempts have been made not only to color a water-soluble resin with an oil-soluble dye or a hydrophobic dye but also to utilize colored microparticles comprising a colorant and a resin covered therewith or colored microparticles in which colored particles comprising a colorant and a resin are further covered with a film-forming resin.
Water resistance and light fastness of a water-based ink are surely improved by these techniques. However, when a residual monomer presents in colored microparticles, it may dissolve a dye to increase the particle diameter or adhere on the nozzle surface to change physical properties resulting in deterioration of ejection stability. Further, it has been proved that the residual monomer may become a radical generating source to accelerate image deterioration when it presents in an image.